Weidong Sheng

A spin-valve device based on dumbbell-shaped graphene nanoislands

Electron transport in a graphene nanoisland with mixed edge profiles is studied by using the spin-unrestricted Hubbard model. The dumbbell-shaped device consists of two hexagonal nanoislands with zigzag edges connected by a armchair nanoribbon. Including the nearest-neighbor hopping and on-site Coulomb repulsion, the self-consistent calculation shows that two distinct ferromagnetic configurations allowed for the proposed device act like ON and OFF states for spin-polarized electrons. Furthermore, the spin-valve effect is demonstrated, as a proof of concept, in the device occupied by one ferromagnetic configuration without resorting to external magnetic or electric fields.

Substrate effects on quasiparticles and excitons in graphene nanoflakes

The effects of substrate on electronic and optical properties of triangular and hexagonal graphene nanoflakes with armchair edges are investigated by using a configuration interaction approach beyond double excitation scheme. The quasiparticle correction to the energy gap and exciton binding energy are found to be dominated by the long-range Coulomb interactions and exhibit similar dependence on the dielectric constant of the substrate, which leads to a cancellation of their contributions to the optical gap. As a result, the optical gaps are shown to be insensitive to the dielectric environment and unexpectedly close to the single-particle gaps.

Origins of optical anisotropy in artificial atoms

The author report on a theoretical study of optical anisotropy in quantum dots. The mechanisms how shape anisotropy and strain field lead to optical anisotropy are identified by an empirical tight-binding approach. The anisotropic structure of quantum dots is shown to impose stronger confinement for the localized p orbitals aligning along the short axis. In self-assembled quantum dots, these orbitals are also seen in a higher potential produced by the strain field. As a result, the valence-band electrons prefer to occupy the orbitals aligning along the long axis, which leads to stronger optical emission polarized along that direction.

Van Hove singularities in graphene nanoflakes

The density of states of graphene diverge at six M points in the Brillouin zone, known as Van Hove singularities. For a finite graphene structure, such as nanoflake, similar singularities are found to emerge in the energy spectrum as highly degenerate states. We investigate these degenerate states in various graphene nanoflakes and show that the existence of the singularities is strongly dependent on the geometry, size, and even edge profile of the structures. While highly degenerate states are seen for all the hexagonal structures, no singularities can be found for any triangular nanoflake which has even number of carbon rings along each side. We further reveal that the nanoflakes with the Van Hove singularities exhibit very different optical absorption spectrum from those structures where the singularities are absent. More interestingly, we find that these highly degenerate states can survive when the structural symmetry is broken by a vacancy. Depending on its location and the sizes of structures, the ...

Electric field driven magnetic phase transition in graphene nanoflakes

Within the framework of Hubbard model, a bowtie-shaped graphene nanoflake is identified to undergo an electric-field induced phase transition from an antiferromagnetic ground state. Unlike the case of half-metallic graphene nanoribbons, the electric field here leads to a non-magnetic state instead of ferromagnetic state after destructing the antiferromagnetic ordering. Because the spin is polarized on different sublattices of the nanodot in the antiferromagnetic phase, the transition occurs when the applied field breaks the sublattice symmetry and induces enough energy splitting among the originally degenerate zero-energy states.

Tunneling transmission in two quantum wires coupled by a magnetically defined barrier

A numerical analysis of an electron waveguide coupler based on two quantum wires coupled by a magnetically defined barrier is presented with the use of the scattering-matrix method. For different geometry parameters and magnetic fields, tunneling transmission spectrum is obtained as a function of the electron energy. Different from that of conventional electron waveguide couplers, the transmission spectrum of the magnetically coupled quantum wires does not have the symmetry with regard to those geometrically symmetrical ports, It was found that the magnetic field in the coupling region drastically enhances the coupling between the two quantum wires for one specific input port while it weakens the coupling for the other input port. The results can be well understood by the formation of the edge states in the magnetically defined barrier region. Thus, whether these edge states couple or decouple to the electronic propagation modes in the two quantum wires, strongly depend on the relative moving directions of electrons in the propagating mode in the input port and the edge states in the magnetic region. This leads to a big difference in transmission coefficients between two quantum wires when injecting electrons via different input ports. Two important coupler specifications, the directivity and uniformity, are calculated which show that the system we considered behaves as a good quantum directional coupler

Polarization of intersubband transitions in self-assembled quantum dots

Intersubband transitions in self-assembled quantum dots are studied by using a multiband tight-binding method. A picture different from that by the single-band effective-mass approximation is presented to reveal the origin of the polarization of the intersubband transitions. It is shown that the symmetry of those minor components from the valence bands in the electronic states accounts for the polarization of the intersubband transitions. A microscopic theory is presented to explain the pattern of symmetry of these minor components in the electronic states. The result is compared with a recent experiment.

g-factor tuning in self-assembled quantum dots

The possibility of electrical tuning of exciton g-factors in self-assembled InAs/GaAs quantum dots is explored theoretically by means of a tight-binding-like effective bond-orbital approach. The electron g-factor in the dots of various sizes is found to exhibit very little change over a broad range of the field strength. In contrast, the ground hole state in the dots of high aspect ratio is seen very sensitive to the applied field, its g-factor even changes the sign with the field. The distinct behavior of the electron and hole g-factors in the presence of electric field is explained in terms of nonzero envelope orbital angular momentum carried by the hole states.

Electrical tuning of exciton g factors in quantum dot molecules: Effect of hole localization

We present a theoretical study of electron and hole g factors in stacked self-assembled InAs/GaAs quantum dots. The exciton ground and first excited states in the quantum dot molecules are found to exhibit opposite resonances in their g factors in the presence of a small vertical electric field, which is very different from the monotonic behavior of their counterparts in single quantum dots. While the g factor of the electronic ground state is seen to have little variation as the applied electric field increases, the relocalization of the hole states in coupled quantum dots is found to account for the resonant behavior of the exciton g factors. Our theoretical result agrees well with a recent experiment.

Microscopic theory of electron cotunneling through quantum dots

A microscopic theory is presented for electron cotunneling through quantum dots in the Coulomb blockade regime. Beyond the semiclassic framework of phenomenological models, a fully quantum mechanical solution for cotunneling of electrons through a one-dimensional quantum dot is obtained by using a quantum transmitting boundary method without any fitting parameters. Elastic and inelastic cotunneling conductance is calculated as a function of the energy of the incident electron. It is revealed that the cotunneling current contains a significant term proportional to V2 (V being the bias voltage) in additional to the well-known V3 term. The result also indicates that the cotunneling conductance exhibits little dependence on the spin configuration of the incident and confined electrons.